As noted in my U.S. Pat. No. 5,200,831, digital printing (the term "printing" is used to encompass both printing and displaying throughout) of gray level has been achieved in a number of different manners. The representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. As an approximation to continuous tone images, pictorial imagery has been represented via binary halftone technologies. In order to record or display a halftone image with a scanning system, one picture element of the recording or display surface consists of a j.times.k matrix of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank, in other words, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency rendition (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another method of producing gray levels is provided by gray level printing. In such a method, each pixel has the capability to render several different dot sizes. The dot size for a pixel is a function of the exposure time provided an LED element corresponding to that pixel. The longer the exposure time, the more toner is attracted to that particular pixel.
There are two major concerns in rendering a continuous tone image for printing: (1) the resolution of image details, and (2) the reproduction of gray scales. In a binary halftone representation scheme, these two fundamental factors compete with each other. The more gray levels that are rendered, the larger is the halftone cell. Consequently, coarse halftone line screens are provided, with the attendant poor image appearance. Hence, a compromise is made in rendering between the selection of line resolution and gray scales in binary halftone printing. However, in gray level halftone printing, one can satisfy both resolution and gray level requirements. In gray level printing, the same number of addressable dots are present, and there is a choice of dot sizes(or alternatively dot densities for dots of the same size), for example, from three dot-sizes or densities of 2 bits/pixel to 15 different dot-sizes or densities of 4 bits/pixel. An image could then be rendered with 133 line screens per inch (5.24 lines per mm) and 128 gray scales of higher quality image. Although providing higher image quality with respect to line resolution and tonal scales, gray level halftoning presents its own dot rendering issues.
My U.S. Pat. No. 5,200,831 addresses the problem that exists in the application of a gray level rendering technique to a document that contains different types of images: text, halftone, and continuous tone. These different types of images create different rendering problems, based on a trade-off between tone scales and detail resolution. For example, with text, the number of tone scales is not as important as providing a smooth text edge, whereas the opposite holds true for continuous tone images. Providing a single type of gray level halftone rendering technique to a document that contains two or more types of images may lead to the production of a document in which one or more of the different types of images are reproduced unsatisfactorily.
When scanning a document, image processing techniques have been applied to convert a gray scale image into an image representation which a printer can accept (either binary format or gray level format). In this scanning process, text areas, line drawing and halftone pictures are indistinguishable from each other, and all appear to be a gray scale image. An improper conversion process creates artifacts in the hardcopy such as a jagged boundary in the text area, or a Moire pattern in the halftone region. To overcome this, intelligent processes have been developed to segment the image into different regions of text, line drawing, and picture. Different conversion processes for the individual segments were then applied to these segments to restore the original document. However, these segmentation and conversion processes unduly complicate the digital copying process.
The unified rendering method and apparatus disclosed in my patent provides gray level printing that satisfactorily reproduces images that contain text, line drawing, halftone and/or continuous tone regions, with different gray dot representations selected for the specific regions.
FIG. 1 illustrates an arrangement which reproduces a document as described in my U.S. Pat. No. 5,200,831. The document 10 can contain different types of images on the same document. For example, document 10 may contain both text and continuous tone areas, and may also contain halftone areas.
The document 10 is scanned in and digitized by a conventional scanner 12, which operates to provide digital signals representative of the densities of the areas of the document 10 corresponding to a pixel. These signals are sent to a memory (or buffer) 14. Under the direction of a controller 16, these signals may be modified and provided as gray level signals through a framestore 17 to a printer 18 and/or a display 20 for each pixel. The printer 18 and/or display 20 will then reproduce the document 10 by energizing each of the individual pixels according to the gray levels as modified (or not modified) by the controller 16. The memory 14, the framestore 17, the printer 18 and the display 20 are of conventional hardware design.
In providing a gray level signal for a specific pixel to be printed, the controller 16 selects between a "mixed dot" type rendering technique and a "fixed threshold" type rendering technique. These two rendering techniques will now be discussed.
In gray level printing, each pixel has the capability to render several different dot sizes, and thus different gray levels. However, instead of simply providing each pixel with an independent gray level, several pixels may be organized together to form a super pixel, or cell. Each of the pixels in a cell is then provided with a gray level. The human visual response integrates the various gray levels of the individual pixels in the cell to a single perceived gray level for the cell. This is similar to the basic concept of binary halftoning. The number of tone scales for a cell is increased greatly, however, due to the number of different gray levels available for each pixel. For example, instead of only the two levels provided in binary halftoning for each pixel, eight levels (including zero) can be provided with gray level printing for each pixel in a cell (3 bits/pixel). When the cell is made up of 4.times.4 pixels, for example, the gray level printing allows 113 different gray shades to be rendered for that cell.
The formation of the dots in the pixels of a cell can be performed in a number of different manners to achieve different desired results. The dots can be formed as "full" dot, "partial" dot, "mixed" dot or fixed dot type to provide gray level halftoning.
FIG. 2 illustrates an example of a 3-bit gray halftone dot layout for a full dot type formation. Also illustrated (greatly magnified) are seven different pixel-dot sizes, corresponding to the relative sizes that each individual pixel-dot can obtain. Note, however, in an electrophotographic system using LEDs as the exposure source primarily dot density rather than dot size may be changed with exposure. For convenience of illustration, the dot densities of the pixels are shown to increase in size. However, such is also intended to illustrate pixels of the same size but having different densities. There are 57 possible gray levels for the exemplary eight element cell 30 shown here. An example of the formation of a cell that is at gray level 12 will now be given.
The pixel circled in level 1, reference numeral 1, is formed to gray level 1 in level 1. (Only one cell will be described, although the pixels of other cells will be changed according to the same layout, as shown in FIG. 2). The dot at this circled pixel grows to higher gray levels as the gray levels for the cell increase from level 1 to level 2 all the way to level 7. One can see that this pixel increases in value from 1 to 7 as the levels increase. If the desired gray level for the cell 30 was 7, then the formation of dots would be completed once the circled pixel has reached the gray level of 7 in level 7. In this example, however, the gray level for the cell 30 is desired to be 12. At gray level 7, the circled pixel has reached its maximum gray level, so that a dot at another pixel must now start forming. This dot starts forming at the pixel indicated with a square around it in level 1, with the numeral 8.
The dot formation process continues, with the dot at this second pixel growing to higher gray levels as the levels again increase from level 1 to level 5. The formation process stops at level 5, since the cell has now reached the gray level value of 12. The halftone cell 30 now contains, as seen in FIG. 3, a dot of gray level 7, and a dot of gray level 5. The extension of this formation process to 57 cell gray levels, including zero, is easy to see from this example.
The full dot type process thus involves forming dots at the highest priority pixels to their maximum allowable gray level before beginning the formation of the dots at the next highest priority pixels.
In the electrophotographic process, the full dot type formation process is favored because it forms stable dots and exhibits less granularity (halftone printing noise). Formation of stable dots implies providing certain minimum exposure at a pixel location to ensure development, particularly in an electrophotographic system. Another method which carries more information detail than full dot, but at the cost of less stable dots, is the partial dot type, described below.
A 3-bit gray halftone dot layout for the partial dot type formation process is shown in FIG. 4. In this process, the cell 34 is built by providing a dot of the same gray level to each pixel in the cell to the extent possible, before building up the dot at any particular pixel to the next larger size. Thus, for a gray level of 6 for the cell 34, the circled pixel in level 1 would have a dot formed at that pixel with a dot gray level of 1 as would also the pixels numbered 2, 3, 4, 5 and 6. For larger gray levels, for example gray level 13, each of the pixels in the cell 34 would be built up to at least dot gray level of 1. The pixels indicated with a square around them in level 2 would be built up to have a dot gray level of 2.
The partial dot formation process can thus be seen to spread out the information over the cell, and therefore carries more information detail than the full dot. It does suffer from less stable dots (particularly in the electrophotographic process) and more granularity, however.
The mixed dot type, discussed below, combines the merits of both the full dot and the partial dot types in gray level halftoning. A number of different processes can be provided to combine the full dot type and the partial dot type, with the specific mixed dot type being chosen based on which renders an image with more smoothness, less graininess, and more image details. Suggested strategies are: 1) build small stable dots in the highlight (toe) region; 2) keep tone response linear in the mid-tone region; 3) reduce dot structure in the shadow (shoulder) region and render more details. Based on these considerations, a specific mixed dot type can be chosen by one of ordinary skill in the art to optimize stable dots, more image detail and less graininess.
An example of a specific mixed dot type 3-bit gray halftone dot layout is illustrated in FIG. 5. As can be seen for cell 36, until cell gray level 41 is reached, the pixels are constrained from growing beyond dot gray level of 5. The pixels grow in a full dot type process, with the pixel circled growing to a dot gray level of 5, with the pixel that is squared then starting to grow in gray level. The assumption underlying the mixed dot structure is that increasing cell gray levels are achieved by having pixels grow to a gray level wherein a stable dot can be formed, using say the electrophotographic process, before moving to another location to grow the next dot in the cell. Once all of the pixels in the cell have attained a gray level size of 5, corresponding to gray level 40, the cell then increases in gray level by using a partial dot type growth process. In other words, each of the pixels in the cell must grow to a dot gray level of 6 before any of the pixels begins growing to a dot gray level of 7.
Another type of rendering technique is a fixed threshold method. In this method each individual pixel is rendered with only limited tone scales. For example, 4 bits/pixel renders 16 different tone shades (including zero). The fixed threshold type renders the highest resolution among the various types, and an edge can be rendered more accurately down to each pixel. The fixed threshold type renders an image with even higher sharpness than the partial dot type since it is not limited by the cell size as is the partial dot type. The problem with the fixed threshold type is that it has less tone scales, so that a false contour could easily be seen in the rendered image. However, the fixed threshold type will provide excellent rendering results on text and halftone originals.
Although any one of the three dot types (full, partial or mixed) could be used to produce a satisfactory continuous tone image, the mixed dot type is the best choice for continuous tone rendering. For scanned text and halftone, the full dot type creates a screen structure in the background of text and a Moire pattern in the halftone. The mixed dot type also creates a screen structure in the background of text and creates a Moire pattern in the halftone, though weaker than that created by the full dot type. As stated above, the fixed threshold type renders well on both text and halftone. The unified rendering technique of the invention described in my aforementioned patent uses both fixed threshold type and mixed dot types according to local image content so that text, halftone and continuous tone images are all reproduced well.
In another embodiment of the invention described in my aforementioned patent, a partial dot is rendered for text and halftone, since text and halftone are usually higher contrast in nature. As with the fixed threshold type, the partial dot type will not cause the Moire pattern in the halftone and will produce a smooth text boundary. The mixed dot, the best of the different dot types for continuous tone images, is still rendered for continuous tone regions.
While the above prior art printing method and apparatus works well, the inventor has noted that further improvements to image quality can be made.
Thus, in the rendering apparatus and method described in U.S. Pat. No. 5,200,831, two distinct dot types are utilized in rendering. They are: partial dot vs. mixed dot or fixed threshold dot vs. mixed dot. The patent discloses that proper switching of a dot size in the rendering will produce a satisfied result on composite documents. However, the partial dot, mixed dot and fixed threshold dot have their distinct dot structures. Abrupt changes during dot switching can cause certain unnatural appearance in the image. It is, thus, an object of the invention to provide an improved rendering apparatus and method which minimizes this dot switching effect.